Microemulsions are used as vehicles for drug delivery. This system can be used by various routes like topical, oral, buccal, sublingual, nasal, vaginal, rectal and intravenous. Advantages of microemulsions are as follows:                These are formed spontaneously on mixing all the components.        Penetration of water soluble drugs is enhanced using water-in-oil microemulsion.        Solubility of lipophilic drugs is enhanced when used as oil-in-water microemulsion.        Release rate of drugs can be controlled.        Molecules can be protected from hydrolysis and oxidation.        Taste masking can be achieved.        
Along with these advantages there are some serious limitations of this drug delivery system. Being fluid in nature, maintaining dose uniformity, handling and transportation are difficult to manage in microemulsions. Although presence of surfactants and co-surfactants impart sufficient stability to this dispersed system, it is affected by environmental conditions like pH, temperature etc.
Hence, to make microemulsions useable for therapeutic purpose, it is necessary to make a drug delivery system from such microemulsions which have dose uniformity, convenience in handling, storage stability and suitability to different routes of administration.
In practical terms, a solid dosage form is preferable to a liquid dosage form in respect of convenience, ease of handling and accurate dosing. Hence, attempts have been made to encapsulate microemulsions in soft gelatine capsules. However, limitation with this kind of delivery is that it can be used only for oral route and not for any other route of administration The dispersed systems in the form of liquids are more prone to instability and excipient incompatibility. Many researchers have attempted to develop powder, re-dispersible emulsion-derived formulations known as dry emulsion from these fluid microemulsions by removing water from water in oil microemulsion, using water soluble or insoluble carriers, by rotary evaporation, lyophilisation or spray drying. However, all of them have some or the other drawback. Common drawback is that the resulting loss of water results in loss of activity of some Active Pharmaceutical Ingredients. This includes, for example, without limitation, macromolecules, particularly enzymes which are sensitive to changes in pH, loss of water, loss of three dimensional structure. This also includes, without limitation, thermolabile Active Pharmaceutical Ingredients, for example, Ketorolac tromethamine, Doxylamine succinate, tetracycline, misoprostol, methylcobalamine, Cholecalciferol, serratiopeptidase, etc.
Moreno et al (Pharmaceutical Research 2001; 18(3):344-351) lyophilized an amphotericin B containing lecithin based o/w microemulsion. But the final formulation was an oily cake and was intended to be used after reconstitution in water. The reconstitution involved addition of measured quantity of water and stirring using magnetic stirrer. After this processing microemulsions were obtained.
So, the work was done to reduce fluidity of microemulsions and not to convert fluid dosage form to solid dosage form while retaining microemulsion properties.
Remidia et al (WO2003051334A2) disclosed a pharmaceutical composition comprising a poorly soluble drug, in powder or microgranular form, comprising an oil/water/oil double microemulsion incorporated into a solid support constituted by a microporous inorganic substance or by an adsorbent colloidal inorganic substance or by a cross-linked swellable in water polymer, wherein said drug is dissolved or dispersed in one or more of the phases of said microemulsion. The composition according to claim 1, formulated with pharmaceutically acceptable excipients or diluents, for use in capsules, pills, sachets and suspensions by incorporation of the o/w/o microemulsion of stage d) into a support in the form of a powder by slowly adding said microemulsion to said support in powder form, maintaining said support under constant mixing/agitation in an equipment selected from high efficiency of mixing granulators, extruders and fluid bed granulators. The solid microporous carrier included, cross linked PVP, silica, silicates, zeolytes, alumina, activated carbon, colloidal silica, magnesium trisilicate, argil, magnesium oxide, talc, CMC starch, CMC cellulose, polystyrene, polymethylmethacrylate etc.
Hong et al (U.S. Pat. No. 6,306,434) disclose a cyclosporin solid-state microemulsion comprising a solidified product consisting essentially of a cyclosporin microemulsion dispersed in an enteric carrier. The enteric polymer containing material was one or more chosen from the group consisting of aqueous methacrylic polymers, hydroxypropyl methylcellulose phthalates, cellulose acetate phthalate and sodium alginate. Here, cyclosporin microemulsion is added to the solution of the enteric carrier, and mixed to make it homogeneous. The solution was then evaporated slowly at low temperature under reduced pressure to remove the solvent completely. Cyclosporin solid-state microemulsion film produced was powdered, and formulated into pharmaceutical preparations, that is, capsule, powder, granule and tablet according to conventional methods. The intermediate product is obtained as film, which was powdered, and formulated using conventional methods into the final pharmaceutical dosage form that is other than a film comprising, without limitations, capsule, powder, granule and tablet. Moreover in this work instead of liquid state microemulsion, a “cyclosporin solid-state microemulsion” i.e. a “solidified product of microemulsion pre-concentrate containing cyclosporin” is used in the sense that they can form microemulsion spontaneously after being dissolved in such external phases as water, etc. Thus, the products of Hong et al do not contain a microemulsion in its liquid form/state.
Retaining compatible solvent at proper pH and other conditions in the microemulsion is needed to retains stability and activity of several pharmaceutical actives, including but not limited to, peptides and enzymes. Hence, a drug delivery system is needed wherein it is a solid dosage form but micro-emulation ingredient in it is not in a dehydrated solid form.